Conventionally, an optical scanner used in a laser beam printer or the like includes, in many cases: a semiconductor laser as a light source; a polygon mirror as an optical deflector; a first imaging optical system for focusing a light beam from the semiconductor laser linearly on the polygon mirror to compensate for the tilt of the surface of the polygon mirror; and a second imaging optical system for forming a uniform spot on a surface to be scanned at constant velocity.
Recently, in order to attain reductions in cost and size, a second imaging optical system formed of one correcting lens has been proposed in JP-A-4-50908, JP-A-62-139520, JP-A-9-281422, JP-A-9-179017, JP-A-8-248308, JP-A-10-148755, or the like.
However, the systems proposed in JP-A-4-50908 and JP-A-62-139520 are not formed considering the asymmetric aberration that is caused by the shift of the reflecting point due to the rotation of the deflecting plane. The system proposed in JP-A-9-179017 does not compensate for the asymmetric aberration caused by the oblique incidence of light onto a deflecting plane. The system proposed in JP-A-9-281422 compensates for the asymmetric aberration caused by the oblique incidence of light onto a deflecting plane by a lens shape alone, resulting in a complicated shape and difficulty in processing.
The system proposed in JP-A-8-248308 compensates for asymmetric constituents of the field curvature in the main scanning direction by the positioning of an optical deflector and the shift of a correcting lens. However, the compensation is insufficient, and in order to compensate for the field curvature in the sub scanning direction, both planes have complicated curved surfaces that are processed with difficulty.
Further, the system proposed in JP-A-10-148755 also can provide an effect compensating for the asymmetric constituents of the field curvature in the main scanning direction by the positioning of an optical deflector, but the compensation is not enough. In addition, since its cross section in the sub scanning direction has a biconvex shape, the numerical aperture in the sub scanning direction on the image surface side varies greatly and thus the beam diameter in the sub scanning direction also varies greatly, which has been a problem.